1. Field of the Invention
The present invention relates to a pneumatic tire having a rib pattern and, more particularly, to a pneumatic tire using ribs having improved wear resistance. Still particularly, the invention relates to a pneumatic tire designed to prevent uneven wear.
2. Description of the Related Art
Main types of uneven wear of a pneumatic tire having a rib pattern are (1) step wear starting from the end of the tire in the direction of the axis of rotation of the tire, the end touching the road surface, and (2) river wear produced in rib ends on both sides of each circumferential groove. Especially, there has been the problem that river wear that is uneven wear produced locally in the circumferential direction of the rib ends is caused easily.
In the past, the above-described step wear, or one kind of uneven wear, has been effectively prevented by forming thin circumferential grooves near the road-contacting shoulder ends of the tire tread. With respect to river wear, a technique of forming multiple short sipes in rib ends circumferentially of the tire at close intervals is known. One end of each of the sipes opens into a circumferential groove.
The process in which the river wear is produced is understood as follows. First, minute steps are formed on the rib ends by lateral forces produced during operation of the tire. Slippage is produced across the steps due to variations in radial diameter, increasing the steps in the lateral direction. The increased steps are grown into the above-described uneven wear. It is quite important to prevent local wear on the rib ends, in order to suppress eruption of river wear. Satisfactory results cannot be obtained only if the short sipes are disposed.
A technique of preventing uneven wear of a rib pattern is disclosed, for example, in JP-A-10-264609. In particular, a tread is partitioned into shoulder ribs on both sides, second ribs located inside the shoulder ribs, and a single center rib located between the second ribs. Generation of rib tears and sipe tears is prevented by limiting the relative widths of the ribs. The progress of uneven wear from the shoulder edges can be hindered even if the wear is in its middle stage or later.
JP-A-2005-132267 proposes a pneumatic tire having excellent wear resistance, uneven wear resistance, and gripping capabilities. The tire has main grooves and thin grooves extending circumferentially while being bent right and left with uniform amplitude. The left and right ribs are circumferentially and axially siped. The tread width is set greater than 0.70 times the tire width. Thus, the relationships of the tread width with the main and thin grooves are stipulated.
JP-A-8-192607 describes a heavy-duty pneumatic tire having excellent uneven wear resistance and wet skidding resistance. The tire has plural zigzag main grooves extending circumferentially and lands partitioned by the main grooves. The lands form rows of multiple blocks split by open sipes. The lands also have closed sipes both ends of each of which terminate inside the blocks.
However, the pneumatic tire relying on the above-described related art can have improved gripping capabilities and steering stability because the ribs are split by each open sipe whose both ends open into the main grooves. However, the rigidity of the ribs tends to vary, and the road holding capability deteriorates. Shearing force produced around the sipes tends to be concentrated in the openings of the sipes at the rib ends. This creates increased strain. As a result, deformation easily occurs. Wear starts from the location where the strain is produced, resulting in cracks. Hence, there is the problem that uneven wear tends to be produced easily.